MONDAY, Aug. 4, 2014 (HealthDay News) -- An experimental serum never before tried in people may have been pivotal in helping treat two Americans stricken with Ebola, according to media reports.

Dr. Kent Brantly, 33, and Nancy Writebol, 59, both contracted the highly fatal virus while working to help infected patients in the West African nation of Liberia. Brantly was flown by specially equipped plane to Atlanta on Saturday for treatment at Emory University Hospital. Writebol is expected to arrive in Atlanta via the same plane on Tuesday and also undergo treatment at Emory.

Brantly had been working with aid agency Samaritan's Purse in Liberia. An unnamed source has told CNN that the U.S. National Institutes of Health contacted Samaritan's Purse and offered three vials of the experimental treatment, called ZMapp, for use in the two patients.

According to the CNN source, Zmapp is a drug being developed by San Diego-based biotech company Mapp Biopharmaceutical. Although the medicine has shown promise in trials involving monkeys, it has never been tested in humans. In the primate trials, four monkeys infected with Ebola survived after getting Zmapp within 24 hours of infection, and two of four other monkeys survived after getting the serum within 48 hours of infection.

CNN said that Brantly woke up July 22 feeling feverish and immediately put himself into isolation. Writebol came down with similar symptoms three days later, and blood tests confirmed that both Americans had contracted Ebola.

Zmapp must be kept frozen. Upon its arrival at the Liberian hospital where Brantly and Writebol were being treated, the serum was allowed to thaw naturally, a process taking 8 to 10 hours, CNN said.

According to a CNN source, Brantly asked that Writebol be given the first dose of Zmapp because he was younger than Writebol and more likely to survive. She agreed but once Brantly's condition deteriorated -- he had difficulty breathing and believed he was dying -- the decision was made to give him the first, thawed dose.

The treatment seems to have worked. Just one hour after receiving intravenous delivery of Zmapp, Brantly made a dramatic recovery, CNN reported. He began breathing on his own and a rash that had once covered his trunk faded away. By the next morning Brantly could take a shower on his own prior to his evacuation to the United States.

Writebol also received Zmapp. Her response after the first dose was not as dramatic as that seen in Brantly, but a second dose was given on Sunday and Writebol has made a significant recovery, CNN said.

How did the two American patients get access to a drug whose safety and effectiveness hadn't yet been tested in humans? Experts note that the U.S. Food and Drug Administration will allow such access in emergency situations based on "compassionate use," although the speed at which the treatment made it to the two patients is remarkable.

Speaking Sunday on NBC's "Meet the Press," Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, agreed that Brantly "seems to be improved from the reports we got earlier."

After being flown from Liberia on the specially outfitted plane, Brantly climbed out of an ambulance with the aid of another person and then walked the short distance to the entrance of Emory University Hospital. Both Brantly and the person who helped him were wearing special biocontainment suits and similar protective gear, NBC News reported.

Experts said the fact that Brantly could walk on his own was an encouraging sign.

Brantly's parents and his wife, Amber, had traveled to the hospital to meet him, but his two small children did not make the trip.

Both Brantly and Writebol had been working at clinics in Liberia, helping victims of an Ebola outbreak that the World Health Organization says has already killed at least 887 people.

The unit at Emory that will care for Brantly and Writebol has been designed for just these types of cases, however. In a statement, Emory said that the unit "has unique equipment and infrastructure that provide an extraordinarily high level of clinical isolation." Staff who work in the unit are specifically trained and practiced in treating "this type of patient," the hospital said.